Dynamic equalizer circuits having a light dependent cell for producing a relatively constant apparent loudness effect



Oct. 25, 1966 K. E. MERCER 3,281,723

DYNAMIC EQUALIZER CIRCUITS HAVING A LIGHT DEPENDENT CELL FOR PRODUCING ARELATIVELY CONSTANT APPARENT LOUDNESS EFFECT Filed Feb. 5, 1964 5Sheets-Sheet 1.

|2 FIG I IO\ PROGRAM SIGNAL SOURCE 12 6 PROGRAM SIGNAL 8 SOURCE 34 :2426y28 W 30 L 1 J AMP INVENTOR.

KENT ELLIOTT MERCER ATTORNEYS LAMP CONTROL Oct. 25, 1966 Filed Feb. 5,1964 K. E. MERCER ,7 DYNAMIC EQUALIZER CIRCUITS HAVING A LIGHT DEPENDENTCELL FOR PRODUCING A RELATIVELY CONSTANT APPARENT LOUDNESS EFFECT 5Sheets-Sheet 2 .JLLI 2 E 55 0 w 3 m 098 ['0 O F *5 S2 1 LI.

a (D E g \l 1 xi g a N8 1 Oct. 25, 1966 K. E. MERCER 3,281,723

DYNAMIC EQUALIZER CIRCUITS HAVING A LIGHT DEPENDENT CELL FOR PRODUCING ARELATIVELY CONSTANT APPARENT LOUDNESS EFFECT 0200mm mwa wm o o 20Zm30mmE s 000 8 000 9 002 09 ON a N awwwmv m N EwNwm v m N awhwmfi n Now o om cm w QE ATTORNEYS United States Patent DYNAMIC EQUALIZERCIRCUITS HAVING A LIGHT DEPENDEN T CELL FOR PRODUCING A RELATIVELYCONSTANT APPARENT LOUDNESS EFFECT Kent Elliott Mercer, New Hyde Park,N.Y., assignor to Fairchild Recording Equipment Corporation, Long IslandCity, N.Y., a corporation of New York Filed Feb. 3, 1964, Ser. No.342,094 11 Claims. (Cl. 333-18) This invention relates to circuits forthe dynamic equalization of variable level program signals and moreparticularly to dynamic equalizer circuits for producing a relativelyconstant apparent loudness etfect by controllably redistributing boththe high and low frequency portions of the signal in a program channelas the program signal level decreases.

Work done by Fletcher and Munson in the area of the effect of thefrequency distribution of program signals on human auditory perceptionhas indicated that as listening levels or program levels are decreased,the perception by humans of the high and low frequency portions of theprogram becomes increasingly difficult. This means that for loud programsignal levels no difficulty is experienced in deriving the high and lowfrequency information but as the program signal level drops, auditoryperception of the program becomes increasingly difficult due to both thedrop in the signal level and the added falling off of auditoryperception to the high and low frequency information. This effect iscalled a decrease in the apparent loudness of the reproduced program,i.e., an increase in the apparent loss of the high and low frequencyportions of the program signals due to hearing losses.

In the reproduction of audio programs it would be desirable to maintaina relatively constant apparent loudness effect so that programs ofdifferent levels could be heard with equal facility. The presentinvention is directed to circuits for compensating for the falling offof high and low frequency auditory perception to low level programsignals by providing a relatively constant apparent loudness effect.This is accomplished by correcting the frequency response of anelectronic audio signal reproducing system to compensate for the humanauditory response in a manner such that the high and low frequencyportions of the program signal are emphasized as the overall programsignal level is decreased. Stated in another way, the circuits of thepresent invention operate to produce loudness or signal responsecompensation curves which are similar to the auditory response curvesderived by Fletcher and Munson. These loudness curves modify the inputprogram signal accordingly, so that the aurally reproduced programsignal has a relatively constant apparent loudness effect to the humanlistener.

In accordance with the invention, circuits are provided for producingthe desired loudness compensation curves in a dynamic manner so that asthe program level drops the loudness curve increases, i.e., the high andlow frequency portions of program signal emphasized more as the programlevel drops, and as the program level increases the loudness curvedecreases. By doing this, a relatively constant apparent loudness effectis produced for substantially all levels of program signal informationwithin the predetermined operating ranges of the circuits. In thevarious embodiments of the invention described herein the programsignals are passed through passive rather than active circuit componentsto produce the desired dynamic loudness compensation curves. Since thesecomponents are passive, no additional distortion or noise is introducedin the final loudness compensated output signals of the circuits.

The circuits of the preferred embodiment of the pres- "ice ent inventionalso utilize light-dependent resistors as one of the components forproducing the desired loudness curves. These light-dependent resistorsoperate in response to the light output provided by a control lamp Whoseoutput is in turn controlled by sensing the level of the input programsignals. In a preferred circuit embodiment the program signals arepassed through a bandpass filter Which provides a constant attenuationfor the mid-range frequencies of an audio program signal. The lightdependent resistors are used to provide a variable attenuation for thehigh and low frequency portions of the signal so that these portions areattenuated less as the program signal level decreases and are attenuatedmore as the program signal level increases. This action produces thedesired loudness curves in response to variable level program signalinputs.

In a preferred embodiment of the invention a continual range of outputloudness curves is available through the use of a threshold controlprovided in the dynamic equalizer circuit. Additionally, the preferredembodiment of the invention also provides means by which the highfrequency and low frequency ends of the loudness compensation curve canbe controlled. This provides the user With Wide latitude in changingloudness curve contours to accommodate program demands and individuallistening tastes.

It is therefore an object of the present invention to provide a dynamicequalizer circuit for producing a relatively constant apparent loudnesseffect.

A further object of the invention is to provide dynamic equalizercircuits for boosting the high and low frequency portions of an audioprogram signal in response to decreasing signal levels of the program.

A further object of the invention is to provide dynamic equalizercircuits utilizing light-dependent resistors in a compensating networkfor producing desired loudness compensation curves in response tovarying level pro-gram input signals.

A further object of the invention is to provide circuits for dynamicallyequalizing the high and low frequency portions of a program signal asthe level of the signal changes in a manner such that as the programlevel drops the loudness compensation curve increases and as the programincreases the loudness compensation curve decreases.

Other objects and advantages of the present invention will become moreapparent upon reference to the following specification and annexeddrawings in which:

FIGURE 1 is a schematic diagram illustrating one circuit for the purposeof illustrating the principles of the present invention;

FIGURE 2 is a schematic diagram of another circuit illustrating theoperating principles of the present invention;

FIGURE 3 is a schematic diagram of the details of a circuit utilizingthe principles of the present invention showing the amplifier circuitfor sensing the level of the input signals and operating the controllamp; and

FIGURE 4 is a diagram illustrating loudness curves obtainable with thecircuit of FIGURE 3.

Referring to FIGURE 1, a source 10 is provided for producing programsignals which preferably include components in the audio frequencyrange, for example in the range from 0 to 20,000 cycles per second. Thesignal source 10 may be any suit-able source, for example, an AM or FMradio or tuner, the output from a monophonic phonograph cartridge or onechannel of a stereo cartridge, an amplifier, preamplifier, film soundtrack, etc. The upper output line from the signal source 10 isillustratively connected to the input of an amplifier 12 while the loweroutput line is connected to a point of reference potential such asground 14. The lower input line of amplifier 12 is also connected to thepoint of reference potential.

Connected between the upper input terminal of the amplifier 12 and thepoint of reference potential 14 is a voltage divider compensatingnetwork formed by a series connected resistor 16, a coil or inductor 18,a capacitor 20 and a resistor 22. This compensating network serves toshunt a portion of the amplifiers input signal from source to the pointof reference potential 14 in a manner which is both frequency selectiveand dependent upon the amplitude of the input signal to amplifier 12.

The compensating network is frequency selective due to the presence ofthe series connected coil 18 and capacitor 20. Components 18 and 20 areselected to form a series tuned resonant circuit which preferably has arelatively wide bandwidth, i.e., relatively low Q. The resonantfrequency of circuit 18, 20 is selected at a point in the midaudiofrequency range, for example, around 1,000 cycles. Since this circuithas a relatively wide bandwidth, its effect will be felt on allfrequencies in the neighborhood of from about 40 cycles up to aboutapproximately 10,000 cycles when the resonant frequency is selected at1,000 cycles. It should be understood that the resonant frequency forthe circuit 18, 20 can be selected as desired and that any type of afrequency selective network or filter may be used.

Shunted across the tuned resonant circuit 18, 20 is a light-dependentresistor 24. The light-dependent resistor is any suitable device, suchas a cadmium sulphide cell for example, whose resistance changes inresponse to the amount of illumination impinging thereon. The cadmiumsulphide cell referred to above has the characteristic that as no lightis impinging thereon it has a relatively high or practically infiniteresistance, in the order of several hundred megohms, while as the lightimpinging thereon increases its resistance decreases in a predeterminedmanner in proportion to the light. The lowest resistance value of thedevice can be under one hundred ohms. Many suitable cadmium sulphidecells and other types of lightdependent devices are currently availableand no further description thereof is believed necessary.

Operating in conjunction with the light-dependent cell 24 is a lightsource or lamp 26 which is powered from the output signal of amplifier12. Lamp 26 is preferably of the incandescent type although a neon lampcould be utilized if it is desired to provide a rectifying effect,actually an averaging effect, on the compensating circuit. Both thelight-dependent cell 24 and the lamp 26 are preferably enclosed in asuitable light-tight housing 28 which prevents changes in externalillumination from affecting the resistance of the cell 24. It should beunderstood that as the amplitude of the output signal from amplifier 12increases, in response to an increasing level of the program signalsfrom source 10, the output illumination of lamp 26 also increasescausing a resultant decrease in the resistance of cell 24.

-range frequencies to the resistor 22 and the point of referencepotential 14. The cell 24 has no effect to shunt the tuned resonantcircuit since its resistance at this time is extremely high. Thus, withlight output from lamp 26 the compensating network 16, 18, 20 and 22 isineffective to those signal components which are not passed by the tunedresonant circuit 18, 20. Under this condition the high and low frequencycomponents of the signal are effectively boosted since the mid-rangecomponents are attenuated by the voltage dividing action.

As the signal level from the source 10 increases, with a resultantincrease in the output of lamp 26, the resistance of the light-dependentcell 24 decreases. This causes shunting of all of the frequencycomponents of the signal from source 10 through cell 24 to the point ofreference potential 14. The result of this action is that the mid-audiofrequencies are subjected to a substantially fixed attenuation,determined primarily by the ratio of resistors 16 and 22, While the highand low frequency components of the same audio signal are subjected to avariable attenuation determined by the input signal level, whichoperates to control the light output of lamp 26, as Well as by the ratioof the resistors 16 and 22.

From the circuit of FIGURE 1 it can be seen that the signal appearingacross the output terminals 6 and 8 will be varied in a mannercorresponding to the Fletcher and Munson curves. This occurs because asthe program signal level from source 10 increases, more and more of thehigh and low frequency components of the program are attenuatedresulting in a decrease in the loudness curve. On the other hand, as theprogram signal level decreases, only the mid-range audio frequencysignals are attenuated, meaning that the high and low frequency portionsof the program are effectively boosted resulting in an increase in theloudness curve.

FIGURE 2 shows another circuit for producing the desired loudness curvesin which the high and low frequency components of a signal areeffectively boosted in response to a decerasing program signal level.Similar reference numerals have been used as in the circuit of FIGURE 1,where applicable. In the circuit of FIGURE 2 the input of a controlamplifier 30 is connected to the output of the source 10 to sense theamplitude level of the program signal. The output of amplifier 30controls the illumination of the lamp 26 which is disposed inlight-receiving relationship to the light-dependent resistor cell 24 inthe light-tight housing 28. Amplifier 12 also receives the programsignal from the source 10 for reproduction at its output terminals 6 and8. Amplifier 12 has an odd number of stages so that a phase shiftedrepresentation of its input signal is present at output at terminal 6.The 180 phase shifted signal is applied as a negative feedback signal atthe input of the amplifier 12 through the series connected resistor 32,the parallel tuned resonant formed by coil 34 and capacitor 36, and theresistor 38. The circuit 34, 36 is made parallel resonant at somefrequency in the midrange and thus will pass the largest negativefeedback signal back to the input of amplifier 12 at the resonantmid-range frequency. As before, the resonant frequency and Q of circuit34, 36 is selected as desired. The parallel resonant circuit 34, 36 isshunted by the light dependent resistor cell 24 so that as itsresistance is decreased, due to increased output of lamp 26, more of thehigher and lower frequency components of the signal at terminals 6 and 8are applied back to the input of amplifier 12 as a negative feedbacksign-a1.

The operation of the circuit of FIGURE 2 is similar to that of FIGURE 1.When the signal level from source 10 is low enough so that the lamp 26is not illuminated by the action of control amplifier 30, then a maximumamplitude negative feedback signal at the mid-audio range is provided bythe parallel tuned resonant circuit 34 and 36 through the resistors 32and 38. This serves to attenuate the mid-range frequency components atoutput terminals 6 and 8 and thus to effectively boost the high and lowfrequency portions of the program signal. the amplitude of the inputsignal from source 10 increases the output of lamp 26 also increases.This decreases the resistance of cell 24 and the low and high frequencyportions of the program signal at the output of amplifier 12 are shuntedthrough the cell 24 back to the input of amplifier 12. This in turndecreases the high and low frequency components of the audio signalpresent at the output of the amplifier 12. Thus, the circuit of FIGURE 2produces a loudness curve whose high and low frequency portions increasein response to a decreasing level program signal and decrease inresponse to increasing program level signals.

It should be understood that both the compensating networks shown inFIGURES 1 and 2 are purely passive, i.e., formed by resistors,capacitors and inductors, so that no noise or distortion is introducedto the progarm signal in either of the circuits. It should also be clearthat the circuits of FIGURES l and 2 are dynamic since the compensationcurve varies continuously in response to the magnitude of the inputsignal.

FIGURE 3 shows details of a circuit utilizing the operating principlesdescribed in FIGURE 1. Here, the program signals are again supplied bythe source whose lower terminal is connected to the point of referencepotential 14. A voltage divider network, similar to that of FIGURE 1 andformed by the series connected resistors 50 and 51, and the series tunedresonant circuit of coil 53 and the capacitor 54, is connected betweenthe upper terminal of source 10 and ground. The coil 53 is shunted by avariable resistor 56 whose function is to control the bass boost of thecompensation curve by varying the effectiveness of the coil 53 to highfrequencies in accordance with the amount of resistance shuntedthereacross. Resistor 56 will attenuate high frequencies and have littleeffect on the lows. If desired, a fixed value resistor (not shown) canbe shunted across capacitor 54 to ground 14 to restrict the maximumamount of low frequency boost.

The frequency selective network 53 and 54 is shunted by a lightdependent resistance device 58 which operates in the manner describedwith respect to FIGURE 1. The output of the circuit of FIGURE 3 is takenacross terminals 6 and 8, the former being connected at the junction ofresistors 50 and 51. As can be seen, a fixed ratio voltage divider ofresistors 50 and 51 is provided for the mid-range audio frequencycomponents at all times and the series resonant network 53, 54 presentsa low impedance to ground 14 for these components.

The resistance of light dependent device 58 is controlled by the outputillumination of an incandescent lamp 60 located within the samelight-tight housing 61. The output of lamp 60 is controlled by theremaining elements of the circuit Whose operation is described below.

The upper output terminal of source 10 is connected to one end of aresistor 62 in the signal control line 63 of the lamp control circuit.The other end of resistor 62 is returned to the ground 14 by anotherlight dependent de vice 64 located in the light-tight housing 61.Resistor 62 and device 64 form a voltage divider network and thefunction of device 64 is to vary its resistance to vary the amplitude ofthe control signal for the control circuit for lamp 60 in accordancewith the amplitude of the signal from source 10. It should be clear thatas the intensity of lamp 60 increases, due to an increase in theamplitude of the program level signal from source 10, that theresistance of device 64 will decrease causing a resultant decrease inthe amplitude of the lamp control signal on line 63. This permitsoperation of the circuit over a relatively extended range before thelamp 60 reaches a condition of saturation, i.e., would respond to nofurther increase in the amplitude of the signal from source 10. Thislatter result is unwanted since it would reduce the effectiveness of thecircuit. Therefore, attenuation of the lamp control signal on line 63 isdesirable.

The lamp control signal on line 63 is applied to the primary winding ofa transformer 65 which is shunted by a resistor 66. The signal inducedinto the secondary of resistor 66 is applied through the slider of apotentiometer 68 connected thereacross and through a resistor 69 to thebase of a conventional PNP common emitter transistor amplifier 70. Thepotentiometer 68 serves as a threshold control since it determines atwhat input signal level on line 63 transistor 70 will conduct. Base biasfor transistor 70 is established through the voltage divider networkformed by resistors 72, 73 and 68. Resistor 72 is connected to asuitable source of B minus potential 77 (not shown) which may be anysuitable rectifier circuit or battery. The secondary of transformer68,also has connected thereto a smoothing circuit formed by the resistor75 and capacitor 76. The emitter circuit of transistor 70 has aconventional emitter bias circuit formed by resistor 78 and capacitor80. Collector bias for transistor 70 is established by a resistor 82connected to resistor 72. A capacitor 83 is connected between thejunction of resistors 72 and 82 and ground 14 to filter the collectorvoltage.

The output signal from the collector of transistor 70 is applied througha capacitor 84 to the base electrode of another conventional PNPtransistor amplifier 85 which has an emitter bias circuit formed byresistor 87 and capacitor 88. The collector of transistor 85 isconnected to the B minus source 77 through the voltage divider formed byresistors 89 and 90. A capacitor 91 is used as a smoothing filter. Anamount of degenerative feedback is provided for the base of transistor85 from the collector through a resistor 92 which is connected betweenthe collector and the base. This increases the input impedance oftransistor 85. The base is returned to ground through a resistor 93.

The output of transistor 85 is taken from the collector electrode andapplied through a capacitor 94 to the base of an NPN transistor 95. Withthe switch 97 in the collector circuit of transistor closed, thetransistor is norm-ally held cut-off by the voltage produced in anemitter bias network formed by resistor 100 and capacitor 102 and thebase bias voltage of resistor 103. The collector is connected to thepoint of reference potential through the switch 97 by a thermistor 98and a resistor 99. The thermistor provides an amount of temperaturecompensation for the circuit.

The collector of transistor 95 is directly connected to the baseelectrode of a PNP transistor 105 whose collector is connected directlyto the B minus supply 77 and whose emitter is returned to ground throughthe capacitor 106. When switch 97 is closed and with transistor 95non-conductive, transistor 105 is biased to cut-off through the voltagedivider comprising the leakage resistance of transistor 95, thermistor98 and resistor 99. When transistor 95 conducts, transistor 105 willalso conduct.

The emitter of transistor 105 is connected directly to the baseelectrode of a PNP transistor 110. The collector of this transistor isalso returned directly to the B minus supply 77 and the emitter isreturned to ground through the lamp 60. The current flowing in theemitter circuit of transistor during conduction will illuminate thelamp. Transistor 110 conducts in response to the conduction oftransistor 105.

The operation of the circuit of FIGURE 3 is as follows. A signal abovethe conduction threshold level of transistor 70, as set by thepotentiometer 68, is amplified by transistors 7 0 and 85 and applied tothe base of normally cut-off transistor 95. This amplified signal tendsto drive transistor 95 towards conduction and in turn causes normallycut-off transistor 105 to conduct. This produces a signal at the emitterelectrode of transistor 105, which signal is averaged or smoothed by thecapacitor 106 and applied to the base of transistor 110 causing it toconduct. The conduction of transistor 110 in turn produces a current inits emitter circuit which makes lamp 60 light. The light output of lamp60 is dependent upon the degree of conduction of transistor 110, whichin turn is dependent upon the amplitude of the control signal on line 63and the amplitude of the signal from source 10.

A metering circuit formed by a meter 112 is provided to give the circuituser a visual indication of the degree of compensation provided by thecircuit. One terminal of the meter is connected to the emitter electrodeof transistor 110 and the other is returned to the B minus supply 77through a variable resistor 114 used to set the meter. A meter shuntresistor 115 is also provided.

The operation of the circuit of FIGURE 3 is similar to that of FIGURE 1.When the program signal level is below the conduction threshold oftransistor 70, lamp 60 is extinguished and a fixed attenuation for themid range frequency components is provided by the resistors 50 and 5This boosts the high and low frequency components of the signal atoutput terminals 6 and 8. Increase in the level of the program signalcauses the lamp 60 to light proportionately causing a decrease in theresistance of cell 58. This produces shunting of the high and lowfrequency components to ground 14 and results in a decrease in theloudness compensation curve. The level of the control signal in line 63is reduced in a similar manner in response to increasing level programsignals by the decrease in resistance of cell 64 in response toincreased light output from lamp 60.

A small amount of idling current is supplied to the lamp 60 through aresistor 120 connected to the base electrode of transistor 95. Thiscurrent is very small and causes the lamp 60 to glow very slightly atall times. Thus, the lamp 60 will be immediately responsive to anycontrol signal above the threshold level and there will be no thermallag as would normally be caused by an incandescent lamp during the timeit takes to heat the filament.

In order to disable the circuit the switch 97 need only be opened. Thisswitch is preferably located on the threshold control resistor 68. Thiscauses transistor 95 to become conductive and in turn drive transistors105 and 110 to maximum conduction. This will cause the lamp 60 toproduce maximum light output and therefore both light dependentresist-ance devices 58 and 64 will have minimum attenuation. The loWresistance of light dependent device 58 shunting the frequency selectivenetwork 53,

54 will almost entirely eliminate the frequency selective effect so thatthe entire frequency range of the input signal will now be attenuatedslightly, rather than just the mid-range. The voltage divider formed byresistors 50 and 51 will still produce an adequate amplitude outputsignal. At the same time, the low resistance of device 64 will shunt thelamp control signal to ground so that the control circuit will berendered relatively inetfective.

FIGURE 4 is a graph showing the amplitude-frequency response of thecircuit of FIGURE 3. It should be noted that as the program signal leveldecreases, the high and low frequency portions of the curves areemphasized. At db, i.e., high program level, there is substantially noloundness compensation While at 50 db, the 1,000 cycle components areabout 10 db down from the low frequency components (20 c.p.s.) and dbdown from the high frequency components (20 kc.). It should beunderstood that the values shown for the loudness curves of FIGURE 4 areillustrative only and that the amount of compensation and contour of thecurves can be varied by suitable selection of circuit components.

The circuits of the present invention have many uses. For example, inscoring movie films or films and tapes for television, when music isfaded to permit narration, the quality of the music usually becomes thindue to the reduced music program level and decrease in the loudnesscurve. By using the present circuits a full-bodied sound can bemaintained during these scoring portions by increasing the loudnesseffect. Also, in complex mixing for phonograph recording the use of thecircuits of the .present invention allows greater definition of lowlevel orchestral passages as scored. Also, when an orchestra fades for avocalist or vocal group, the circuits maintain a better quality soundfor the music portion of the record.

The circuits also find use in situations using filmed or tapedtelevision commercials where the commercial appears to be louder thanthe normal program which it accompanies. This problem can be overcome toa considerable degree by using the circuits of the present invention onthe program sources and not on the commercials.

The present invention can also be used effectively in background musicapplications to maintain proper balance of the reproduced sound over awide range of reproduction levels used. Additionally, the circuits canbe installed in all types of sound reproduction systems such as motionpicture sound reproducing systems, stadium and auditorium speechreinforcement systems, etc., to provide a balanced listener response.

While preferred embodiments of the invention have been described above,it will be understood that these are illustrative only, and theinvention is limited solely by the appended claims.

What is claimed is:

1. Apparatus for attenuating on a frequency selective basis programsignals from a source generating variable amplitude level input audiofrequency program signals having high, low and mid-range frequencycomponents comprising: output means,

frequency responsive means connected to said output means forattenuating said mid-range frequency components of said program signalsrelative to said high and low frequency components, a control lamp,means for connecting said control lamp to said source and responsive tothe amplitude level of the program signals for controlling the lightoutput of said lamp,

light dependent resistor means disposed in light receiving relationshipto said control lamp and responsive to the amount of light producedthereby for producing a variable resistance, and means connecting saidlight dependent resistor means in shunt With said frequency responsivemeans for reducing the attenuating effect of said attenuating means asthe resistance of said light dependent resistor means decreases therebycontrolling the attenuation of the mid-range frequency components ofsaid program signals appearing at said output means relative to saidhigh and low frequency components.

2. Apparatus as set forth in claim 1 and further comprising thresholdmeans connected to said lamp light output control means for producinglight output from said lamp in response to program level signals above apredetermined amplitude.

3. Apparatus as set forth in claim 1 and further comprising a secondlight dependent resistor means disposed in light receiving relationshipto said control lamp and connected to said lamp light output controlmeans for varying the light output of the lamp.

4. Apparatus as in claim 1 further comprising amplifier means forconnection to said output means and to said source, and means connectingsaid attenuating means between the input and output of said amplifiermeans to provide a feedback signal for controlling the gain of saidamplifier means.

5. Apparatus as in claim 1 wherein said attenuating means comprises atuned resonant circuit including an inductor and a capacitor and saidlight dependent resistor is connected in shunt with said tuned resonantcircuit.

6. Apparatus as in claim 5 further comprising amplifier means forconnection to said output means and to said source, and means connectingsaid resonant circuit between the input and output of said amplifiermeans to provide a feedback signal for controlling the gain of saidamplifier means.

7. Apparatus as in claim 5 wherein said inductor and capacitor areconnected in parallel.

8. Apparatus as in claim 5 wherein said inductor and capacitor areconnected in series.

9. Apparatus as in claim 8 wherein said tuned resonant circuit andshunting light dependent resistor are connected in series with saidoutput means.

10. Apparatus for attenuating on a frequency selective basis programsignals from a source generating variable amplitude level input audiofrequency program signals 9 10 having high, low and mid-range frequencycomponents level of the program signals -for producing increasingcomprising: lamp light output in response to increasing amplitudeamplifier means having an input for receiving said prolevel programsignals.

gram signals from said source and an output, 11. Apparatus as in claim10 wherein said feedback frequency responsive feedback means connectedbe- 5 means includes a tuned resonant circuit formed by a partween theoutput and input of said amplifier means allel connected capacitor andinductor and means confor applying a degenerative feedback signal backto necting said light dependent resistor in shunt with said the input ofsaid amplifier in which the mid-range res nant Cir uit. frequencycomponents of the program signals are of increased amplitude relative tothe high and low 10 References Clted by the Exammel' frequencycomponents, UNITED STATES PATENTS control lamp 1,788,035 1/1931Stevenson 179-1 light dependent resistor means disposed in light receiv-1,938,256 12/1933 Jacobs 179-1 mg relationship to S'aId control lamp andconnected 2 171 048 8/1939 Rockwell to said frequency responsivefeedback means so that 15 2177050 10/1939 Bartels 179 1 said feedbackmeans and said light dependent re- 2680232 6/1954 Clams T sistor meanspass increasing amounts of the high 2:9O0:609 8/1959 Estkowski and lowfrequency components of the program signals back to the input of saidamplifier means in ELI LIEBERMAN primary Examiner response to increasedprogram signal levels,

and means for connection to said source and connected GENSLER AssistantExaminer to said control lamp and responsive to the amplitude UNITEDSTATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,281,723October 25, 1966 Kent Elliott Mercer It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 3, line 60, strike out "ner in proportion to the light. Thelowest resistance value"; column 4, line 29, for "decerasing" readdecreasing column 5, line 7, for "progarm" read program Signed andsealed this 5th day of September 1967.

(SEAL) EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDEROfficer

1. APPARATUS FOR ATTENUATING ON A FREQUENCY SELECTIVE BASIS PROGRAMSIGNALS FROM A SOURCE GENERATING VARIABLE AMPLITUDE LEVEL INPUT AUDIOFREQUENCY PROGRAM SIGNALS HAVING HIGH, LOW AND MID-RANGE FREQUENCYCOMPONENTS COMPRISING: OUTPUT MEANS, FREQUENCY RESPONSIVE MEANSCONNECTED TO SAID OUTPUT MEANS FOR ATTENUATING SAID MID-RANGE FREQUENCYCOMPONENTS OF SAID PROGRAM SIGNALS RELATIVE TO SAID HIGH AND LOWFREQUENCY COMPONENTS, A CONTROL LAMP, MEANS FOR CONNECTING SAID CONTROLLAMP TO SAID SOURCE AND RESPONSIVE TO THE AMPLITUDE LEVEL OF THE PROGRAMSIGNALS FOR CONTROLLING THE LIGHT OUTPUT OF SAID LAMP, LIGHT DEPENDENTRESISTOR MEANS DISPOSED IN LIGHT RECEIVING RELATIONSHIP TO SAID CONTROLLAMP AND RESPONSIVE TO THE AMOUNT OF LIGHT PRODUCED THEREBY FORPRODUCING A VARIABLE RESISTANCE, AND MEANS CONNECTING SAID LIGHTDEPENDENT RESISTOR MEANS IN SHUNT WITH SAID FREQUENCY RESPONSIVE MEANSFOR REDUCING THE ATTENUATING EFFECT OF SAID ATTENUATING MEANS AS THERESISTANCE OF SAID LIGHT DEPENDENT RESISTOR MEANS DECREASES THEREBYCONTROLLING THE ATTENUATION OF THE MID-RANGE FREQUENCY COMPONENTS OFSAID PROGRAM SIGNALS APPEARING AT SAID OUTPUT MEANS RELATIVE TO SAIDHIGH AND LOW FREQUENCY COMPONENTS.